The present invention relates to circuits for providing duplex telecommunication over communication lines in the context of a communication system, and more particularly, to a digital echo cancelling speakerphone.
One aspect of face-to-face human communication is the ability of two parties to both talk and be heard at the same time. The ability to both speak and be heard at the same time has presented technical complications in telecommunication systems. For example, with respect to radio frequency carrier wave transmissions, if two parties to a conversation transmit at the same time, the signals will interfere with each other causing beat frequency oscillations, feedback and the like. Systems which solve this problem and thus allow the parties to the conversation to both speak and be heard at the same time are referred to as duplex systems.
Until the introduction of speaker telephones, virtually all telephones were duplex systems. Generally, such telephone systems during this period included the equivalent of a pair of variable resistance carbon microphones and a pair of electromagnetic earphones, connected in series. As one caller spoke over the line, a diaphragm coupled to a carbon powder compartment in the microphone caused successive compressions to be exerted against the carbon powder, thus varying the electrical resistance of the compartment. This in turn varied the current passing through the series circuit thereby modulating the current passing through the series combination to generate the telephone signal. This modulated signal, modulated at the actual frequency of the voice signal being transmitted, was present at the telephone set of both participants to the telephone conversation. This system, which continues in use in principle unchanged from the original instruments developed by Bell in the 1870""s, inherently has a duplex characteristic. Duplex communication is achieved because the audio frequencies involved do not cause unacceptable interference with each other and because the gain of the feedback loop between the carbon microphone and the earphone is less than unity.
With the advent of speaker telephones, it became necessary to introduce into the telephone instrument an audio amplifier for amplifying the audio signals received from the telephone central office to a level that is sufficient to drive a loudspeaker. This immediately presented the problem of preventing feedback between a microphone adjusted for sensitivity to the voice of a person, who is not speaking directly and proximately into it, while making the system unresponsive to audio signals introduced into the environment by the loudspeaker which could be very close to the microphone. To somewhat better understand this problem, it must be kept in mind that the telephone is a two-wire system used to carry both the transmitted and received signal. If the transmitted signal is thus allowed to be amplified by the amplifier which amplifies the received signal which is also carried on the same two wires, ambient noise will be amplified and feedback oscillations are likely to ensue at normal levels of speaker amplitude which results generally in loop instability.
One approach to this problem was embodied in speaker telephone systems that included separate microphones and loudspeakers, both of which had some directional characteristic designed to ensure that audio on the loudspeaker would be loud enough for the telephone user to hear while at the same time having, perhaps, less audio field strength at the location of the microphone. In addition, the solution involved a microphone whose directional sensitivity characteristic was directed toward the mouth of the individual using the system with minimal sensitivity in the vicinity of the speaker. Thus, design objectives involved reducing the gain of the feedback loop between the microphone and the speaker to less than unity, with the volume control for the system set at a level that would allow easy intelligibility of the signal.
Such an approach does not, in principle, provide a commercially acceptable level of performance. For example, such an approach imposes limits on the location of the parties to the conversation, maximum loudspeaker volume and minimum user voice levels. Another approach to this problem, and one which is more widespread in modern communication systems, is the sacrifice of duplex operation for relatively trouble-free speaker telephone operation. Generally, these systems incorporate an electronic switch which turns off the speaker when the user is speaking or disables the local microphone when the party at the other end of the telephone is speaking. Alternatively, loss can be inserted into the receive path when the user is speaking and loss can be inserted in the transmit path when the party at the other end is speaking. Such inserted losses are generally complementary in nature, that is, the loss is completely removed from one path and inserted into the other path. This loss is referred to xe2x80x9cswitched lossxe2x80x9d in the speaker telephone system and its amount is such that the overall loop gain is less than unity. In such speaker telephone systems, the degree of switched loss that is required varies directly with the magnitude of the acoustic and line echo path coupling (that is, the signal amplitude in such paths).
The determination of whether a particular telephone is in the transmit or receive mode has been made in such systems, for example, by monitoring the magnitude of the signals at each end of the telephone conversation and selecting the stronger of the two. One approach to the determination of whether a particular speakerphone is in the transmit, receive, or idle state is disclosed in U.S. Pat. No. 5,007,046 of Erving et al., the disclosure of which is hereby incorporated by reference in its entirety.
More recently, a so-called xe2x80x9chybridxe2x80x9d technology has been implemented which resolves some of the difficulties attendant with the speakerphone feedback problem, by reducing the line feedback coupling. The hybrid electronic circuit operates by approximating the complex impedance of the telephone system to produce a cancellation signal. When added to the signal on the telephone system (comprising both the transmitted and received signal), a third signal results which (ideally) includes only the received signal. The third signal is then sent to the audio amplifier and loudspeaker of the speaker telephone system generally free of echo.
Echo cancelling speaker telephones provide yet another solution to the problem by using acoustic and xe2x80x9chybridxe2x80x9d or line echo cancelers to reduce the acoustic and hybrid coupling, respectively. In particular, the hybrid echo canceler identifies the linear component of the hybrid echo path impulse response and uses this to digitally filter the line out signal in order to create a replica of the hybrid echo. This replica signal is then subtracted from the line in signal thus reducing the effect of the hybrid echo. Similarly, the acoustic echo canceler identifies the linear component of the acoustic echo path impulse response and uses this to digitally filter the speaker out signal in order to create a replica of the acoustic echo which is then subtracted from the mike signal to reduce the effect of the acoustic echo. With reduced acoustic and hybrid echo, less switched loss is required, and this enhances the extent of full-duplex communication.
A problem remains, however, in maximizing overall loop gain when the tap state of a canceler has an abnormal setting. Our invention addresses that need using an echo cancelling tap profile to continually adjust the amount of loss in a signal path to maintain loop stability or to better isolate a coupled signal.
Our invention concerns, in one aspect, a method for controlling the amount of loss in signal paths of a communication system. Our method includes the steps of sampling the impulse response of an echo cancelling circuit, detecting an abnormal signal condition by comparing the impulse response to a reference, and inserting loss into a first signal path when an abnormal signal condition is detected. A peak value within the impulse response, or an interval thereof, may be identified so that the detecting step causes the peak value to be compared to the reference, which may be the peak value from a prior sampled impulse response, e.g., an immediately preceding impulse response or interval thereof.
Our invention also concerns a controller for adjusting the amount of loss in the signal paths of a communication system of the type which has an echo cancelling circuit including a memory. The controller includes an adjustable attenuator in the signal path having a control signal input. The controller also has a memory element which stores a reference and a comparator having one input connected to the memory of the echo canceler circuit which samples the impulse response, another input connected to the memory which stores the reference, and an output connected to the control signal input of the attenuator. In operation, the elements of the controller cooperate to compare the signals at the inputs to the comparator so as to output a control signal which selectively adjusts the attenuator to insert or remove loss in the signal path.
Further, our invention concerns a controller for maintaining loop stability in the signal path of an echo cancelling communication system circuit is disclosed. The controller in accordance with this aspect of our invention includes a program running on a digital signal processor within the communication system, with the program analyzing the impulse response of the echo canceler so as to generate a control signal. The controller also has a call programmable gain attenuator in the signal path which is responsive to the control signal from the program to selectively insert or remove loss into the signal path.
In another aspect, our invention concerns a voice command recognition system which utilizes an impulse analyzing control circuit as described above.